1. Field of the Invention
The present invention relates to a pulse width modulation control system, in particular to the pulse width modulation control system which is capable of preventing arm short phenomenon, performing precision control of a three-phase inverter and a converter by securing dead time in pulse width modulation control by setting two reference voltage values and its renewal start point as start point of an up/down counter.
2. Description of the Prior Art
Pulse width modulation is a method which changes pulse width in accordance with amplitude of signal wave. In particular, sine wave pulse width modulation is a method which orderly changes width of output pulse in order to get current flow of a motor similar to sine wave, torque ripple of a motor is small in low frequency extent, accordingly the method is widely used in recent years.
The conventional pulse width modulation control system comprises an up/down counter which outputs chopping wave (Carrier Signal), a comparator which compares the chopping wave outputted from the up/down counter with three-phase reference signal having sine wave pattern, and an output control unit which is inputted the signal outputted from the comparator and outputs load voltage to a operating circuit of a motor.
The conventional pulse width modulation control system will now be described with reference to the accompanying drawings.
FIG. 1A, FIG. 1B and FIG. 1C are wave form diagrams of input/output signal of the conventional pulse width modulation control system. FIG. 1A depicts three-phase sine wave used as reference signal and the chopping wave outputted from the counter. FIG. 1B depicts load voltage generated in accordance with the comparison between the three-phase sine wave and chopping wave.
FIG. 2 is a circuit diagram of the conventional three-phase inverter operation circuit, it comprises six switches, two switches between the six switches connected to same load (same phase between the three-phase) perform complemented switch operation. In other words, positive signal between the load voltage signal is provided to the load through S1 switch, negative signal between the load voltage signal is provided to the load through S4 switch. Herein the negative signal is ON after the positive signal is OFF, or the negative signal is OFF after the positive signal is ON.
According to this, current flows on load L1 as depicted in FIG. 1C. Current flows on load L2 and load L3 in accordance with the same switch operation of the load L1.
Herein, a certain amount of time is required to be the negative signal ON after the positive signal is OFF, or to be the positive signal ON after the negative signal is OFF, the required time is dead time. When the dead time does not exist or is not continued a certain time, arm short phenomenon is generated.
FIG. 3A and FIG. 3B are wave form diagrams of chopping wave having upper carrier and lower carrier. As depicted in FIGS. 3A and 3B, a dual carrier method uses two kinds of chopping signal (Carrier Signal). The method is used in order to prevent arm short phenomenon. FIG. 3A illustrates renewal of reference voltage value at minimum voltage value of the chopping wave, and FIG. 3B illustrates renewal of reference voltage value at maximum voltage value of the chopping wave.
However, when reference signal exists between the upper carrier and lower carrier, arm short phenomenon may be generated in the dual carrier method due to lack of the dead time. To solve above mentioned problem, the dual carrier method renews reference voltage value at the next maximum voltage value or performs over modulation operation.
Nevertheless, precision control of a inverter and converter is impossible because the positive signal always outputs on signal regardless of wave pattern of the chopping wave and the negative signal always outputs off signal.
It is an object of the present invention to provide a pulse width modulation control system which is capable of securing dead time, and performing precision control of a three-phase inverter or converter by renewing reference voltage value not at maximum or minimum voltage value of chopping wave, but at start point of a counter.
The other object of the present invention is to provide the pulse width modulation control system which is capable of generating automatically two reference voltage values on single chopping wave without using a dual carrier method.
To achieve above-mentioned objects, the present invention comprises a counter which outputs chopping wave crossed at maximum voltage value and minimum voltage value after receiving clock input, an adder/subtracter which increases/decreases voltage signal outputted from the counter, a reference voltage value generation unit which outputs two reference voltage value having different value to single chopping wave after receiving voltage signal outputted from the adder/subtracter, a comparator which separately compares reference voltage value generated by the reference voltage value generation unit with voltage value outputted from the adder/subtracter, and an output control unit which outputs PWM (Pulse Width Modulation) positive signal (SPWM+) and PWM (Pulse Width Modulation) negative signal (SPWMxe2x88x92) in accordance with output voltage value of the comparator.